In this kind of plasma generating apparatus due to electric discharge, the commercial frequency voltage or the high frequency voltage is raised up with high frequency transformer, so that the plasma is generated by applying its raised voltage between the opposed electrodes.
For example, in a plasma surface processing apparatus shown in Japanese Patent Laid-Open No. 2001-297898 of the following Patent Document 1, the plasma generating apparatus is disclosed in FIG. 18. This plasma generating apparatus consists of a power supply circuit 100, a high frequency transformer 103, and a pair of electrodes 104, 105. Said power supply circuit 100 applies the high frequency voltage to the primary coil 101 of said high frequency transformer 103. At the secondary coil 102 of high frequency transformer 103, the raised high frequency voltage of about several thousand to ten thousand V occurs, and this high frequency voltage is applied between a pair of electrodes 104, 105. As a result, a plasma is generated by an electric discharge occurred between said pair of electrodes 104, 105. An air pump P generates an air flow by receiving the power supply from said power supply circuit 100. This air flow is supplied between a pair of electrodes 104, 105 and the plasma generated between electrodes is blown on the surface of object to be processed 106. By means of this plasma irradiation, the surface treatment of object to be processed 106 is performed. In addition, the condensers 107, 110 for limiting the discharge current are interposed between said secondary coil 102 and each electrode 104, 105. The resistance 109 connected in parallel to condensers 107, 110 is disposed to make the charged voltage remaining in condensers 107, 110 discharge immediately after stop of discharge and to prevent electric shocks due to its remaining electric charge.
Said plasma generating apparatus is applied to a plasma surface treatment device of object to be processed. However, since the possible range of plasma treatment is restricted essentially in the surface treatment device by single electrode pair, a plurality of plasma surface treatment devices are required to perform the treatment more in depth and more effectively. However, a method that a plurality of plasma surface treatment devices are employed increases in cost and a problem that the scale of treatment device becomes larger occurs. As one of countermeasure of this problem, there are methods that the power supplying circuit portion such as power supply circuit and high frequency transformer in the plasma generating apparatus is used in common, and the electric power is supplied to plural pairs of electrode, so that it is considered that the device generating the wider and larger plasma can be developed with small size and low cost.
FIG. 17 shows a common use example of power supplying circuit portion. The same reference numerals are used for the same components as FIG. 18. The common electrode 112 is disposed at the ground side of secondary coil 102, and two electrodes 111, 113 are arranged opposed to said common electrode. The condensers 114, 115 are interposed between each electrode and said secondary coil 102. According to such constitution, the power supply circuit 100 and the high frequency transformer 103 can be used in common for two pairs of electrode, but there were the following problems concerning the real plasma generation.
For example, when the electric discharge occurs between electrode 111 and common electrode 112, the electric discharge of condenser 114 advances with it. By this electric discharge, the electrical potential A of one output end of the secondary coil 102 to which condenser 114 was connected falls to several tens to 10 V, so that voltage drop occurs. As a result of this voltage drop, since the charged electricity of condenser 115 at side of another one electrode 113 flows to the electrode 111 and is lost, there was a problem that the electric discharge between electrode 111 and common electrode 112 is not induced.
[Patent Document 1] Japanese Patent Laid-Open No. 2001-297898